In order to switch off short-circuit currents in secondary distribution systems, for the most part switching devices are employed that have one or more current paths which, in turn, encompass stationary and movable switching contact elements. Here, the movable switching contact elements can be jointly moved between a closed position, in which the movable and stationary switching contact elements that are associated with each other make contact with each other, and an open position, in which an air break is formed between each of the movable and stationary switching contact elements that are associated with each other. As soon as the movable switching contact elements are moved under load—that is to say, are moved under a current flow—into the open position, (breaking) arcs are created along the air breaks. The duration of the arcs determines the switching time since the current flow between the switching contact elements is maintained. Moreover, the arcs release a large quantity of heat that leads to thermal destruction of the switching contact elements and thus to a shortening of the service life of the switching device. Consequently, there is a need to quench the arcs as quickly as possible, which can be done by arc-quenching devices such as, for example, arc splitters, arc-quenching plates or deion plates. These quenching devices split the arcs into individual partial arcs; the arcs are reliably quenched when the arc voltages are higher than the driving voltages.
For alternating-current applications, the quenching of the arcs is facilitated in that the current has a natural zero passage. When high (short-circuit) currents have to be switched off, however, an arc-back can occur after the zero passage; however, the arcs formed at high currents, in turn, create such a large self-magnetic field that they are automatically deflected towards the arc-quenching devices and are ultimately quenched.
When it comes to switching devices for direct-current applications, no automatic interruption of the arc occurs as is the case with the zero passage of alternating current. Consequently, for direct-current applications, so-called blow-out magnets are employed that generate a magnetic field whose strength and orientation exert a deflecting force (Lorentz force) on the arcs, thus deflecting the arcs towards the arc-quenching devices. The arcs are stretched, cooled and split into partial arcs in the arc-quenching devices, as a result of which they are quenched.
Switching devices of the above-mentioned type for alternating-current applications are described, for example, in DE 103 52 934 B4, DE 102 12 948 B4, DE 20 2005 007 878 U1, EP 1 594 148 A1, EP 0 980 085 B1 and EP 0 217 106 B1.
Typically, a distinction is made between alternating-current and direct-current switching devices. Whereas alternating-current switching devices of the one-pole or multi-pole type can be produced inexpensively in large quantities, direct-current switching devices in the form of one-pole or two-pole switching devices are manufactured in considerably smaller production runs. Consequently, direct-current switching devices, some with a prescribed direction of incoming supply, are special devices. The use of renewable sources of energy such as, for instance, solar energy, fuel cells, battery series and so forth calls for more switching devices that have a direct-current switching capability as well as an isolating function in the low and medium current ranges at voltages of up to about 1000 V.